Lots of food is irradiated as part of the processing. Seafood, shellfish, spices, vegetables. Hospitals have irradiators for fresh blood to zap it before transfusions.
Cesium 137 is commonly used, it lasts longer than Cobalt-60. One issue with it is it is produced as cesium chloride, which i soluble. What probably happened here is that a food irradiator developed a small leak and Cs-Cl got out into the processing line.
The world got dosed with Cs137 when Chernobyl melted, but it is spread very thinly.
The Chernobyl disaster released an estimated 80,000 terabecquerels (TBq) of Cesium-137 into the environment. Since 1945, man-made activities have released an estimated one million terabecquerels (TBq) of Cesium-137 into the environment. So Chernobyl represents just 8% of the total quantity of man-released Cs-137.
Regards,